The present invention relates to optical amplifiers and specifically to optical amplifiers having an approximately constant output level with respect to input level.
Communications systems and specifically telecommunications systems often comprise optical amplifiers. These optical amplifiers are typically used to transmit digitally encoded optical signals over long distances. A popular type of optical amplifier used in communication systems is an optical amplifier comprising an optical fiber doped with a rare earth element, such as erbium. The erbium-doped fiber (EDF) is xe2x80x9cpumpedxe2x80x9d with light at a selected wavelength, e.g., 980 nm, to provide amplification or gain at wavelengths within the low loss window of the optical fiber. However, one limitation of any rare-earth doped optical fiber amplifier is unequal gain over a range of frequencies or optical channels of interest, as well as for various input signal strengths.
Accordingly, optical amplifiers attempting to provide uniform spectral gain have been developed. In particular, optical amplifiers including an active optical filter (gain compensation filter) have been developed. Typically, these active optical filters selectively attenuate high gain wavelengths, while passing low gain wavelengths. However, optical amplifiers incorporating these gain compensation filters are unable to receive input optical signals over a wide range of power levels while maintaining substantially uniform gain at each wavelength.
Other attempts employ a multi-stage optical fiber amplifier for providing gain equalization. Typically, the amplifier comprises several stages of amplification where each stage comprises an amplifying fiber having a different gain spectrum. For example, prior art systems are known to comprise two stages of amplification, which are pumped separately. Each stage has a different dopant composition to provide a different gain spectrum. However, equalization is limited to a relatively narrow bandwidth.
Previous attempts to compensate for unequal gain of EDF amplifiers are typically complicated and require sophisticated calibration. Also, previous attempts comprise active components, which require additional power. Further, previous attempts tend to be adversely affected by fluctuations in environmental conditions, such as temperature and humidity. These adverse affects include more noise injected into the optical signal and increased bit error rate. (The bit error rate (BER) is a measure defined as the number of error bits divided by the total number of bits received in a given time period.) The performance of prior art systems is also known to deteriorate over time, thus requiring multiple iterations of calibration. Thus, a need exists for a robust optical amplifier, not requiring additional power to accommodate unequal gain, providing a low BER over time and fluctuating environmental-conditions.
An optical amplifier for amplifying at least one input signal and providing at least-one output signal includes a nonlinear light absorber for passively controlling the level of the output signal(s). The nonlinear light absorber provides an approximately constant output signal level independent of the level of the input signal.